refracting telescope

A simple refracting telescope has a similar optical system to a microscope. It consists of an objective lens [1] (of focal length fo) and an eyepiece lens [2] (of focal length fe). Parallel light from a distant object converges to form an inverted image [3]. Light rays then appear to come from the large upside-down image [4].

A telescope that uses a lens or system of lenses to gather light from a distant object and bring it to
a focus. The biggest drawbacks of such an instrument is chromatic
aberration, which degrades the sharpness and accuracy of the image by
effects such as fringing and false color,
and the difficulty and expense of making large lenses. An apochromatic
refractor avoids the problem of chromatic aberration to a large
extent by bringing all primary colors to focus at the same point. Refractors
are no longer built for professional use.

In 1756, John Dollond introduced the first achromatic lenses consisting of a combination
of two lenses made from crown and flint glass. By the early 1800s, Joseph Fraunhofer succeeded in applying Dollond's
technique to lenses of substantial size. This ushered in the era of giant
refracting telescopes which reached its height at the end of the 19th century
with the instruments of Alvan Clark, including
those at the Lick Observatory and Yerkes
Observatory, both of which date back to roughly 1890.

So long as lenses are made of glass, the 40-inch (1-meter) at Yerkes represents
the practical limit in size of refractors. Although glass blanks larger
than 40 inches in diameter have been cast during the past 50 years, they
are seldom, if ever, sufficiently free of internal defects to make satisfactory
lenses. Even if an acceptable blank were obtained, the resulting lens, supported
only by its edge, would distort so badly from its own weight as to be optically
useless.

Before large reflecting telescopes took over in the 20th century, refractors
were generally preferred for detailed observations of the planets since,
having an enclosed tube, they produced a steadier image.